Liquid discharge devices, in which a plurality of pressure chambers to be filled with a liquid are arranged in a planar direction on one surface of a board, a nozzle for discharging the liquid as a liquid drop is formed for each of the pressure chambers on the opposite surface of the board, each of the pressure chambers and the corresponding nozzle are interconnected by a communication path to be filled with a liquid, and a piezoelectric actuator including a piezoelectric element is disposed on the one surface, on which the pressure chambers are formed, of the board, have been widely used as piezoelectric ink jet heads in recording devices utilizing ink jet recording systems, for example, ink jet printers and ink jet plotters.
In the above-mentioned liquid discharge device, when the piezoelectric actuator is vibrated so as to repeat a state where it is deflected in the thickness direction and a state where the deflection is released by applying a predetermined driving voltage pulse to the piezoelectric element with the pressure chamber and the communication path respectively filled with the liquids, the volume of the pressure chamber is increased or decreased with the vibration so that the liquid in the pressure chamber vibrates. The vibration is transmitted to the nozzle through the liquid in the communication path so that a meniscus of the liquid formed in the nozzle vibrates. A part of the liquid forming the meniscus is separated as a liquid drop with the vibration, and the liquid drop is discharged from the nozzle. In the case of the piezoelectric ink jet head, the liquid drop (ink drop) discharged from the nozzle flies to a paper surface disposed opposite to the nozzle, to reach the paper surface, so that dots are formed on the paper surface.
Conventionally, the communication path has been generally formed so as to have a substantially constant opening area, considering that the vibration of the liquid in the pressure chamber is transmitted to the meniscus in the nozzle as smoothly as possible. For example, Patent Document 1 describes a liquid discharge device in which a communication path is formed so as to have a predetermined opening area from an opening on the side of a pressure chamber to a position connecting with a nozzle, and the nozzle is formed in a tapered shape such that its opening area gradually decreases to its tip from a position connecting with the communication path.
However, consideration by the inventors have proved that in a conventional liquid discharge device in which the opening area of a communication path is made substantially constant, as described in Patent Document 1, when a piezoelectric actuator is driven, to discharge a liquid drop from a nozzle by a mechanism previously described, a liquid drop having a previously designed volume and flying speed cannot be discharged from the nozzle because micro vibration of a liquid is generated in the communication path, and the micro vibration is overlapped with vibration of a liquid in a pressure chamber so that the volume and the flying speed of a formed liquid drop vary.
As a cause of this, the inventors have considered that a part of vibration transmitted to the liquid in the communication path is transmitted to the meniscus of the liquid in the nozzle, as previously described, while the remainder thereof is reflected toward the pressure chamber in the vicinity of an inlet to the nozzle because the opening area of the communication path is larger than the opening area of the nozzle. That is, the remainder of the vibration reflected in the vicinity of the inlet to the nozzle is repeatedly reflected between the vicinity of the inlet to the nozzle and a surface opposite the inlet to the communication path on an inner wall surface of the pressure chamber to generate a standing wave, to micro-vibrate the liquid in the communication path.
The period of the micro vibration is mainly defined by the distance between the opposite surfaces, between which the vibration is repeatedly reflected, for example, and is a small value that is a small fraction of the period of the vibration of the liquid generated by driving the piezoelectric actuator. When the micro vibration is overlapped with the vibration of the liquid generated by driving the piezoelectric actuator, however, pressure for discharge, which is applied to the meniscus of the liquid in the nozzle, becomes excessively high or excessively low depending on the amount of shift in phase between both the vibrations. Therefore, the volume and the flying speed of the formed liquid drop vary, as previously described.
In a case where the micro vibration is overlapped with the vibration of the liquid generated by driving the piezoelectric actuator so that the pressure for discharge, which is applied to the meniscus of the liquid in the nozzle, becomes excessively higher than a normal value, for example, when the piezoelectric actuator is driven to discharge the liquid drop from the nozzle, a so-called head high-speed drop being minuter and having a higher flying speed than a predetermined liquid drop is easily discharged as the first drop.
The amount of shift in phase between the vibration of the liquid generated by driving the piezoelectric actuator and the micro vibration is mainly determined by the length of the communication path, for example. Therefore, the volume and the flying speed of a liquid drop discharged from one nozzle do not drastically vary while the liquid discharge device is employed. However, the volumes and the flying speeds of liquid drops discharged from a plurality of nozzles formed on the one board in the liquid discharge device easily vary for each of the nozzles. In the case of the piezoelectric ink jet head, the head high-speed drop is generated, and the volumes and the flying speeds of the liquid drops discharged from the plurality of nozzles vary, so that the image quality of a formed image is reduced.
Patent Document 1: Japanese Unexamined Patent Publication No. 2005-144917 (Paragraph [0029], FIGS. 1 and 2)